Method and apparatus for applying a flux
This application is a Continuation of application Ser. No. 07/265,233, filed Oct. 31, 1988, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a soldering method and an apparatus for applying flux materials onto a geometrically defined joint to be soldered. 
     BACKGROUND INFORMATION 
     Methods and apparatus for applying flux materials are known in large numbers. In that context, spray methods and spray apparatus are also included which allow a small flux material application and cause essentially no capillary effect, especially for endangered components such as open relay contacts, etc. 
     It is a disadvantage of known flux material spray methods and apparatus that the flux material cannot be applied in a directed or geometrically defined and limited manner. Generally, components or defined surfaces must be masked due to the arising vapors which in its result, is very costly. Normally, the areas surrounding the surfaces to be sprayed, and the parts located thereon, are considerably fouled even with costly masking measures. 
     OBJECTS OF THE INVENTION 
     It is therefore the object of the invention to provide a method and an apparatus for applying a soldering flux onto a geometrically exactly defined small area point forming a joint to be soldered, whereby a quantity as uniform as possible is to be applied in the spacially defined region. 
     SUMMARY OF THE INVENTION 
     The foregoing object has been achieved according to the invention provides in that the flux material is first ultrasonically atomized in a pressureless or almost pressureless manner and that then the flux material mist is transported to the joint to be soldered and is applied to the joint by means of a directed airstream. 
     The pressureless or almost pressureless ultrasonic atomization of the flux material permits generating a flux material mist substantially without any air turbulence. It is formed directly in the area of the atomizer element. The directed airstream then serves to apply the flux substance in the flux material mist onto the joint to be soldered, whereby the directed airstream transports the flux material mist from the atomizer element to the joint to be soldered for achieving the desired deposition, without soiling areas outside the joint to be soldered. 
     Especially flux materials with a very low solid content, such as on the order of magnitude of 1%, are very well suited for an ultrasonic atomization and in many cases are completely sufficient for the soldering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail with reference to an example embodiment shown in the accompanying drawings, wherein: 
     FIG. 1 shows a principle sketch of the apparatus for applying a flux material with an ultrasonic atomizer; and 
     FIG. 2 shows a schematic top view of the apparatus for ultrasonically atomizing. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE OF THE INVENTION 
     An apparatus 1 for applying a flux material onto a geometrically defined joint to be soldered, for example, onto a circuit board which is not shown in the figures, comprises an ultrasonic atomizer 2 which includes an oscillator 3, a vibration generator integrated into a housing 4, and an atomizer element 5. The atomizer element 5, is, for example, trumpet-shaped, and comprises a pipe 6 with a conically expanding free end. By means of the oscillator 3 via a connecting conduit 7, the vibration generator in the housing 4 is excited to vibrate or oscillate and to transmit the vibrations to the atomizer element 5, so that the liquid located there is excited into corresponding oscillations and in the case of ultrasonic oscillations, is atomized or misted. 
     The flux material enters the atomizer element 5 with its trumpet-shaped free end and its inlet pipe 6 through a pipe or hose conduit 8 attached to the housing 4. The other end of the pipe or hose conduit 8 is connected to a supply container 9 for flux material. 
     The ultrasonic atomization of the flux material at the atomizer element 5 is to be achieved in a manner which is as close to pressure-free as possible. For this reason, the supply container 9 is provided with an overflow 10 which is arranged in such a manner that the liquid level 11 of the flux material in the supply container 9 is located at the same level or almost the same level as the trumpet-shaped opening of the atomizer element 5. According to the principle of communicating pipes, flux material then flows out of the supply container 9 through the conduit 8 and a connecting conduit located in the housing 4 into the pipe-shaped atomizer element 5 and is then available without pressure at the trumpet-shaped end of the atomizer element 5. 
     The supply container 9 is further connected by another conduit 12 to a pump 13 which sucks flux material from a second supply container 14 and pumps it into the supply container 9. The overflow 10 is connected to the second supply container 14 so that the pump 13 can pump flux material in a closed circuit loop through the conduit 12 to the supply container 9, whereby simultaneously the unneeded excess flows back to the supply container 14 and thus to the pump 13 while maintaining a constant liquid level in the container 9. 
     Further, FIG. 1 shows schematically with dashed lines a transport arrangement 15 for a work piece carrier 16. The transport arrangement 15 is located over or above the ultrasonic atomizer 2 of which the atomizer element 5 is directed upwardly with its trumpet-shaped opening. In order to apply flux material onto a work piece secured to the work piece carrier 16, but not shown in FIG. 1, the work piece carrier 16 is moved over the ultrasonic atomizer 2 and remains in this position until the flux material application has been achieved. Since only a flux material mist is formed at the trumpet-shaped opening of the atomizer element 5 during the ultrasonic atomization, the actual transport and application of the flux material onto the work piece is finally achieved by means of an airstream which entrains the flux material mist in the area of the trumpet-shaped opening of the atomizer element 5 and blows it to the work piece. In order to produce this airstream, an air pressure conduit 17 leads to the housing 4 of the ultrasonic atomizer 2 and is connected to a nozzle arrangement or an annular nozzle 18 (FIG. 2) which concentrically surrounds the atomizer element 5. The annular nozzle 18 forms a hollow cross-section airstream which surrounds a flux mist cloud formed by said atomizer element 5. The hollow constant airstream carries the flux mist cloud to a small area point to be soldered. 
     It is to be understood that the supply container 9 is arranged laterally next to the transport arrangement 15 so that it does not interfere with the latter. The conduit 8 leading from the supply container 9 to the housing 4 of the ultrasonic atomizer 2 is furthermore as flexible as possible so that the ultrasonic atomizer 2 is not only usable in a stationary position, but rather may itself be also movably supported and is movable relative to the transport arrangement 15, preferably perpendicularly to the motion direction of the latter. 
     Such an arrangement with an ultrasonic atomizer 2 which itself is movable perpendicularly to the motion direction 19 of a transport arrangement 15, is also shown by dashed lines in FIG. 2. Here, the ultrasonic atomizer 2 together with a blower arrangement including the annular nozzle 18 and a pressurized air supply or pressurized air conduit 17 are arranged on a slide-type carrier 20 and movable with the carrier 20 perpendicularly to the motion direction 19 of the transport arrangement 15 for the work pieces supported on the carrier 16. Details of the slide-type carrier 20 are not shown in the figures, however, it reaches through an elongated hole 21 in a floor plate 22 and is integrated into a sealing band 23 which covers the elongated hole 21. The sealing band 23 is made of an upper strand or run and a lower strand or run and is guided around idler rollers 24. The upper strand 25 lies on the top surface of the floor plate 22 and covers with its edges the elongated hole 21 in the floor plate 22. 
     A circulatingly running belt drive 26 arranged parallel to the sealing band 23 with an engaging catch 27 which directly engages the slide-type carrier 20, is provided as a drive for the slidetype carrier 20. A motor 28 serves for controlling the belt drive 26 in the respectively desired motion direction corresponding to the arrow 29. The two direction reversal points may be adjustable in a known manner so that the working width of the ultrasonic atomizer 2 moving back and forth in the crosswise direction may be adapted to the width of the work piece to be provided with a flux material application, for example, a backside of a circuit board to be soldered and may be oriented relative to the transport arrangement 15 for the work piece moving continuously forward. 
     Finally, a droplet collection tub 30 is provided for the ultrasonic atomizer 2 in the arrangement shown in FIG. 2. It is also attached to the carrier 20 and surrounds the atomizer element 5. 
     Although the invention has been described with reference to specific example embodiments it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims.